Abstract

A computational study is described, which addresses how well spatially resolved time-integrated Kα images recorded in intense laser-plasma experiments correlate with the distribution of “hot” (>1 MeV) electrons as they propagate through the target. The hot electron angular distribution leaving the laser-plasma region is critically important for many applications such as Fast Ignition or laser based x-ray sources; and Kα images are commonly used as a diagnostic. It is found that Kα images can easily mislead due to refluxing and other effects. Using the particle-in-cell code LSP, it is shown that a Kα image is not solely determined by the initial population of forward directed hot electrons, but rather also depends upon “delayed”hot electrons, and in fact continues to evolve long after the end of the laser interaction. Of particular note, there is a population of hot electrons created during the laser-plasma interaction that acquire a velocity direction opposite that of the laser and subsequently reflux off the front surface of the target, deflect when they encounter magnetic fields in the laser-plasma region, and then traverse the target in a wide spatial distribution. These delayed fast electrons create significant features in the Kα time-integrated images. Electrons refluxing from the sides and the back of the target are also found to play a significant role in forming the final Kα image. The relative contribution of these processes is found to vary depending on depth within target. These effects make efforts to find simple correlations between Kα images and, for example, Fast Ignition relevant parameters prone to error. Suggestions for future target design are provided.

Received 15 February 2011Accepted 14 June 2011Published online 22 July 2011

Acknowledgments:

We acknowledge useful discussion with Scott Wilks and Dale Welch. This work was performed with support from DOE under contract nos. DE-FG02-05ER54834 and DE-AC52-07NA27344, and allocations of computing time from the Ohio Supercomputer Center and the Lawrence Livermore National Laboratory (LLNL) Institutional Computing Grand Challenge program.

Article outline:I. INTRODUCTIONII. NUMERICAL SIMULATIONS USING LSP AND Kα IMAGES AT TWO DIFFERENT DEPTHSIII. FRONT SURFACE REFLUXINGIV. LONG PULSE RESULTS AND ANALYSISV. SUMMARY